heater problem

hey guys i got a 1988 gmc sierra and the heat only comes out from the top of the dash....... it dosnt come from another vent .. vary confusing cuz its been like this since i bought it any ideas on whats wrong?

The HVAC system dampers are actuated by vacuum motors. Lacking any vacuum, all the dampers will return to a full outside air/full defrost mode (for safety). The vacuum used to operate the dampers is likely not present.

The supply is usually a vacuum line attached directly to the intake. This will route to a 'T' fitting and check valve, where the two remaining lines will route to the vehicle interior and to a vacuum reservoir. There should be a vacuum reservoir under the hood, sometimes bolted to an inner fender or the underside of the hood itself. Any damage to this reservoir will cause a loss of control vacuum.

If the check valve leaks, vacuum to the damper controls may be present at idle or low loads, but fail at higher loads.

The vacuum line into the interior is usually a small, hard plastic line routed into the cab with a wire harness.

Once you establish a good vacuum supply to the controls, the dampers should work as designed. There is a possibility that the vacuum control valve itself has failed, or an interior vacuum line has failed, but you should be able to hear vacuum leakage inside the cab if that's the case.